add background color to subdefs of transparent tiff. (#4376) #344

Job	Run time
build phraseanet-db / build	30s
build phraseanet-elasticsearch / build	26s
build phraseanet-nginx / build	2m 56s
build phraseanet-fpm / build	5m 27s
build phraseanet-setup / build	3m 53s
build phraseanet-worker / build	4m 56s
unit test	4m 25s
	22m 33s

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